1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to a method for updating information on a sleep identifier allocated to a mobile station, and a system using the same.
2. Description of the Related Art
Active research on the 4th generation (4G) communication system, which is the next generation communication system, is being conducted to provide users with high-rate services supporting various Quality-of-Services (QoSs). Recently, many studies of the 4 G communication system are being made to support a high-speed service capable of guaranteeing the mobility and the QoS in a BWA communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system. The typical example BWA system is the Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system or the IEEE 802.16e communication system.
The IEEE 802.16a communication system and the IEEE 802.16e communication system utilize Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) to support a broadband transmission network for a physical channel of the wireless MAN system. More specifically, the IEEE 802.16a communication system does not consider the mobility of subscriber stations (SSs), i.e., is directed to fixed MSs and a unicell structure. However, the IEEE 802.16e communication system considers the mobility of the SSs. Herein, an SS having the mobility will be referred to as a mobile station (MS).
The IEEE 802.16e communication system, as it considers the mobility of MSs, has a problem of high MS power consumption compared with other systems. As a typical method for minimizing the MS power consumption, a sleep mode and an awake mode between the MS and a base station (BS) have been proposed. In this case, the MS performs a ranging operation of periodically adjusting a timing offset, a frequency offset, and power with the BS in order to cope with a change in the quality of a channel to the BS. In particular, a periodic ranging operation is very important for the IEEE 802.16e communication system as it considers the mobility of MSs.
FIG. 1 is a diagram illustrating a sleep mode operation of a conventional IEEE 802.16e communication system. However, before a description of FIG. 1 is given, it should be noted that the sleep mode has been proposed to minimize MS power consumption in an idle interval for which no packet data is transmitted, during transmission of packet data. That is, in the sleep mode, an MS and a BS simultaneously transition to the sleep mode to minimize the MS power consumption in the idle interval in which no packet data is transmitted.
Generally, the interval for which no packet data is transmitted is equal in operation to the interval for which packet data is transmitted. Because such an operation is unreasonable, the sleep mode has been proposed. If there is packet data to transmit in the sleep mode, both the BS and the MS must simultaneously transition to the awake mode to transmit and receive packet data.
The sleep mode has been proposed to minimize the power consumption and inter-channel interference. However, because packet data is affected by traffic, the traffic characteristic and transmission type characteristic must be taken into consideration in the sleep mode operation.
Referring to FIG. 1, reference numeral 110 denotes a packet data generation format. The packet data generation format 110 includes a plurality of ON intervals and a plurality of OFF intervals. The ON intervals are burst intervals for which packet data, i.e., traffic, is generated, and the OFF intervals are idle intervals for which no traffic is generated. According to the traffic generation pattern, the MS and the BS alternately transition (mode change) to the sleep mode and the awake mode, thereby minimizing power consumption of the MS and canceling interference between channel signals.
Reference numeral 120 denotes a mode change format for the MS and the BS. The mode change format 120 for the MS and the BS includes a plurality of awake modes and a plurality of sleep modes. The awake modes represent the modes in which traffic is generated, and in the awake modes, actual transmission and reception of packet data is achieved. The sleep modes represent the modes in which no traffic is generated, and in the sleep modes, actual transmission and reception of the packet data is not achieved.
Reference numeral 130 denotes an MS power level format that represents a power level of the MS according to the packet data generation format 110 and the mode change format 120. In the MS power level format 130, an MS power level for the awake mode is represented by ‘K’, and an MS power level for the sleep mode is represented by ‘M’. Comparing the MS power level K for the awake mode with the MS power level M for the sleep mode, the M is much less than the K. That is, in the sleep mode, because there is no packet data transmission/reception, the power consumption is insignificant.
A description will now be made herein below of the schemes currently proposed to support the sleep mode operation in the IEEE 802.16e communication system. However, before a description of the schemes currently proposed in the IEEE 802.16e communication system is given, the following preconditions will be described.
In order to transition to the sleep mode, the MS must receive a mode change approval from the BS. The BS transmits an approval for transition to the sleep mode to the MS and then transmits packet data. The BS must transmit information indicating the presence of transmission packet data to the MS, during a listening interval of the MS. In this case, the MS must awake from the sleep mode and determine if there is packet data to be transmitted thereto from the BS.
If it is determined that there is packet data to be transmitted thereto from the BS, the MS transitions to the awake mode and receives packet data form the BS. However, if it is determined that there is no packet data to be transmitted thereto from the BS, the MS can either return to the sleep mode or maintain the awake mode.
Parameters for Supporting Sleep Mode and Awake Mode Operations
A description will now be made of the parameters required to support the sleep mode and awake mode operations currently proposed in the IEEE 802.16e communication system.
(1) Sleep Identifier (SLPID)
The SLPID is a value that an MS is allocated through a Sleep-Response (SLP-RSP) message for transitioning from the awake mode to the sleep mode, and is uniquely allocated only to the MSs in the sleep mode. That is, the SLPID is an ID used for identifying an MS in the sleep mode including the listening interval, and if the corresponding MS makes a mode change from the sleep mode to the awake mode, the SLPID previously allocated to the MS is returned to the BS so that another MS wanting to transition to the sleep mode can reuse the SLPID through the SLP-RSP message. Commonly, the SLPID has a 10-bit size, and thus can be used for identifying a total of 1024 MSs in sleep mode operation.
(2) Sleep Interval
The sleep interval is an interval that a BS allocates to an MS at the request of the MS, and represents the time interval for which the MS maintains the sleep mode until a listening interval starts after the MS makes a mode change from the awake mode to the sleep mode. That is, the sleep interval is defined as the total time interval for which the MS is in the sleep mode.
The MS can continuously maintain the sleep mode if there is no data transmitted from the BS even after the sleep interval. In this case, the MS updates the sleep interval while increasing the sleep interval using predetermined initial-sleep window and final-sleep window values. The initial-sleep window value represents an initial minimum value of the sleep interval, and the final-sleep window value represents a final maximum value of the sleep interval. The initial-sleep window value and the final-sleep window value can be represented by the number of frames.
The listening interval is an interval that a BS allocates to an MS at the request of the MS. The listening interval corresponds to the time interval for which the MS temporarily awakes to receive downlink messages such as a traffic indication (TRF-IND) message from the BS during the sleep mode operation, and in the listening interval, the MS can receive the downlink messages in synchronism with a downlink signal from the BS. The TRF-IND message indicates if there is traffic to be transmitted to the MS, i.e., indicates if there is packet data.
The MS continuously waits for the reception of the TRF-IND message for the listening interval. If a bit indicating the MS in an SLPID bitmap included in the TRF-IND message represents a positive indication value, the MS continuously maintains the awake mode, thereby transitioning to the awake mode. However, if the bit indicating the MS in the SLPID bitmap included in the TRF-IND message represents a negative indication value, the MS transitions back to the sleep mode.
(3) Sleep Interval Update Algorithm
Upon transitioning to the sleep mode, the MS determines a sleep interval, regarding the minimum window value as the minimum sleep mode interval. Thereafter, if the MS wakes up from the sleep mode for the listening interval and determines that there is no packet data to be transmitted from the BS, the MS sets the sleep interval to an interval that is 2 times the previous sleep interval, and continuously maintains the sleep mode. For example, if the minimum window value is ‘2’, the MS sets the sleep interval to an interval of 2 frames, and then maintains the sleep mode for the 2 frames. After a lapse of the 2 frames, the MS awakes from the sleep mode for the listening interval, and determines if a TRF-IND message is received. If the TRF-IND message is not received, i.e., if there is no packet data to be transmitted thereto from the BS, the MS sets the sleep interval to a 4-frame interval, which is 2 times the 2-frame interval, and then maintains the sleep mode for the 4 frames. Accordingly, the sleep interval can increase between the minimum window value and the maximum window value.
Messages for Supporting Sleep Mode and Awake Mode Operation
A description will now be made of the messages currently defined to support the sleep mode and awake mode operations in the IEEE 802.16e communication system.
(1) Sleep-Request (SLP-REQ) Message
The SLP-REQ message, a message transmitted from an MS to a BS, is used when the MS requests a mode change to the sleep mode. The SLP-REQ message includes the parameters, or information elements (IEs), required by the MS to operate in the sleep mode. A format of the SLP-REQ message is shown below in Table 1.
TABLE 1SyntaxSizeNotesSLP-REQ_Message_Format( ) {Management message type = 508 bitsInitial-sleep window6 bitsFinal-sleep window10 bits Listening interval6 bitsReserved2 bits}
The SLP-REQ message is a dedicated message transmitted on the basis of a connection ID (CID) of an MS, and IEs of the SLP-REQ message include a Management Message Type, an Initial-Sleep Window, a Final-Sleep Window, and a Listening Interval. The Management Message Type indicates a type of the current transmission message, and Management Message Type=50 indicates the SLP-REQ message. The Initial-Sleep Window indicates a requested start value for the sleep interval (measured in frames), and the Final-Sleep Window indicates a requested stop value for the sleep interval (measured in frames). That is, as described with reference to the sleep interval update algorithm, the sleep interval can be updated between the initial-window value and the final-window value. The Listening Interval indicates a requested listening interval (measured in frames), and the listening interval can also be represented by the number of frames.
(2) Sleep-Response (SLP-RSP) Message
The SLP-RSP message, a response message to the SLP-REQ message, can be used to approve or deny a mode change to the sleep mode requested by the MS, or can be used to indicate an unsolicited instruction. The SLP-RSP message includes IEs needed by the MS to operate in the sleep mode, and a format of the SLP-RSP message is shown in Table 2.
TABLE 2SyntaxSizeNotes MOB-SLP-RSP_Message_Format( ) {Management message type = 51 8 bitsSleep-approved 1 bit0: Sleep-mode request denied1: Sleep-mode request approvedIf (Sleep-approved == 0) { 1 bit0: The MS may retransmit theMOB_SLPREQ message after thetime duration (REQ-duration) givenby the BS in this message1: The MS shall not retransmit theMOB_SLPREQ message and shallawait theMOB_SLPRSP message from theBSREQ-duration 4 bitsTime duration for case whereAfter-REQ-action value is 0reserved 2 bits } else {  Start frame  initial-sleep windows 6 bits  final-sleep windows10 bits  listening interval 6 bits  SLPID10 bits }}
The SLP-RSP message is also a dedicated message transmitted on the basis of a Basic CID of an MS, and IEs of the SLP-RSP message illustrated in Table 2 will be described below.
Management Message Type indicates a type of the current transmission message, and Management Message Type=51 indicates the SLP-RSP message. Sleep-Approved is expressed with 1 bit, wherein Sleep-Approved=0 indicates sleep-mode request denied and Sleep-Approved=1 indicates sleep-mode request approved. More specifically, Sleep-Approved=0 indicates that a mode change to the sleep mode requested by the MS is denied by the BS. Upon receiving the denial, the MS transmits the SLP-REQ message to the BS according to conditions, or waits for the reception of the SLP-RSP message indicating unsolicited instruction from the BS.
For Sleep-Approved=1, the SLP-RSP message includes Start Frame, Initial-Sleep Window, Final-Sleep Window, Listening Interval, and SLPID. For Sleep-Approved=0, the SLP-RSP message includes After-REQ-Action and REQ-Duration. The Start Frame value indicates a frame value up to the time when the MS enters the first sleep interval, and does not include the frame where the SLP-RSP message is received (the number of frames (not including the frame in which the message has been received) until the MS shall enter the first sleep interval). That is, the MS transitions to the sleep mode after a lapse of frames corresponding to the start frame value from the next frame after the frame over which the SLP-RSP message has been received. The SLPID is used for identifying MSs in the sleep mode, and can be used for identifying a total of 1024 MSs in the sleep mode.
As described above, the Initial-Sleep Window value indicates a start value for the sleep interval (measured in frames), and the listening interval value indicates a value for the listening interval (measured in frames). The Final-Sleep Window value indicates a stop value for the sleep interval (measured in frames). The After-REQ-action value indicates an operation that the MS, whose request to the sleep mode has been denied, must perform.
(3) Traffic Indication (TRF-IND) Message
The TRF-IND message, a message transmitted from a BS to an MS for the listening interval, indicates the presence of packet data to be transmitted from the BS to the MS. A format of the TRF-IND message is shown below in Table 3.
TABLE 3SyntaxSizeNotes MOB-TRF-IND_Message_Format( ){Management Message Type = 52 8 bitsFMT 1 bit0 = SLPID basedformat1 = CID basedformatif(FMT==0){ Byte of SLPID bitmap 8 bits SLPID bitmapVariable} else { Num-pos 7 bitsNumber of CIDs onthe positiveindication list for(i=0 ; i<Num-pos ; i++){  Short Basic CID12 bitBasic CID } while (!byte_boundary){  Padding bits1padding for bytealignment }}}
The TRF-IND message is a broadcasting message that is transmitted on a broadcasting basis, unlike the SLP-REQ message and the SLP-RSP message. The TRF-IND message indicates the presence/absence of packet data to be transmitted from the BS to a particular MS, and the MS encodes the broadcasted TRF-IND message for the listening interval, and determines whether to transition to the awake mode, or to transition back to the sleep mode, according to the decoding result.
When determining to transition to the awake mode, the MS detects frame synchronization, and if a corresponding frame sequence number is not identical to a frame sequence number expected by the MS, the MS can request retransmission of the packet data lost in the awake mode. Otherwise, if the MS fails to receive the TRF-IND message for the listening interval, or if a value indicating positive indication is not included in the TRF-IND message even though the TRF-IND message is received, the MS may return to the sleep mode.
For the IEs in the TRF-IND message, Management Message Type indicates a type of the current transmission message, and Management Message Type=52 indicates the TRF-IND message. FMT indicates whether to use an SLPID or a Basic CID of an MS in the process of indicating the presence/absence of the traffic to be transmitted to the MS in the sleep mode. When the SLPID is used for the indication, the SLPID bitmap indicates a set of indication indexes allocated bit by bit to each of the SLPIDs allocated to MSs to identify the MSs that has transitioned to the sleep mode. That is, the SLPID bitmap indicates a group of bits allocated bit by bit to each MS, for (maximum value-1) SLPIDs among the SLPIDs allocated to the MS in the sleep mode. The SLPID bitmap may be allocated dummy bits through byte alignment.
One bit allocated to the MS indicates the presence/absence of data to be transmitted from the BS to the corresponding MS. Therefore, an MS in the sleep mode reads a bit mapped to an SLPID that was allocated during a mode change to the sleep mode from the TRF-IND message received for the listening interval, and if the read bit indicates a positive indication value, i.e., a value of ‘1’, the MS continuously maintains the awake mode, thereby transitioning to the awake mode. Otherwise, if the allocated bit indicates a negative value, i.e., a value of ‘0’, the MS transitions back to the sleep mode.
The BS sequentially allocates SLPIDs to MSs entering the sleep mode in the order of an SLPID with the smaller number among unallocated SLPIDs. During the sleep mode, the MS continuously uses the fixed SLPID allocated from the BS in the initial phase of the sleep mode until it returns to the awake mode.
In this case, each MS that has entered the sleep mode must read the SLPID bitmap from its beginning until a corresponding part where its own SLPID is located, in order to determine the present/absence of packet data transmitted thereto. Because the SLPID that the MS is allocated is fixed to the initially allocated number, if there are many unallocated empty SLPIDs in the SLPID bitmap, there is a considerable waste of resources and time required for reading SLPIDs. That is, increasing the number of MSs entering the sleep mode increases SLPID numbers allocated to the MSs. Therefore, an MS with a greater SLPID number, as its allocated SLPID number is fixed, has a long processing time for reading and processing the SLPID bitmap up to its traffic SLPID. In addition, though the number of MSs that have actually entered the sleep mode is not large, if a difference between the least SLPID and the greatest SLPID among the SLPIDs allocated to the MSs is great, the SLPID bitmap excessively increases.